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description

Intestinal flagellated protozoans and termites exhibit
obligative mutualism, a strict interdependency, in which the protozoans digest the wood ingested by the termites; neither partner can survive under natural conditions without the other.

eukaryotic cells

...cells are thought to have originated as separate organisms that took up residence inside other cells. Eventually neither organism was able to survive without the other—a situation called obligative symbiosis.

yucca plants and moths

...tubular flowers. These appear to be pollinated only by certain hawk moths (family Sphingidae) with very long “tongues” (proboscises). The mutualism of the yucca moths and yucca plants is obligate in that the moth larvae feed only in yucca fruits and the latter can develop only from moth-pollinated flowers. The female yucca moth has special tentacles on the mouthparts, with which it...